This invention relates to a method and device for the production of hydrogen by water splitting as well as combining oxygen from the water splitting to produce syngas and optionally subjecting the syngas to a water gas shift reaction to produce carbon dioxide and hydrogen.
Global environmental concerns have ignited research to develop energy generation technologies which have minimal ecological damage. Concerns of global climate change are driving nations to develop electric power generation technologies and transportation technologies which reduce carbon dioxide emissions. Hydrogen is considered the fuel of choice for both the electric power and transportation industries.
The need to generate ever larger amounts of hydrogen is clear. Outside of direct coal liquefaction, other major industrial activities, such as petroleum refining, also require hydrogen. Collectively, petroleum refining and the production of ammonia and methanol consume approximately 95 percent of all deliberately manufactured hydrogen in the United States. As crude oil quality deteriorates, and as more stringent restrictions on sulfur, nitrogen and aromatics are imposed, the need for more hydrogen for the refining industry will increase.
Hydrogen production, as a consequence of other processes, is significant. A number of industries requiring hydrogen produce effluents containing significant amounts of unused hydrogen. However, this hydrogen requires clean-up prior to re-use. Furthermore, hydrogen is produced from the combustion of oil, methane, coal, and other petroleum-based materials. However, this hydrogen must be separated from other combustion gases, namely carbon dioxide, in order to be of use.
Petroleum refineries currently use cryogenics, pressure swing adsorption (PSA), and membrane systems for hydrogen recovery. However, each of these technologies have their limitations. For example, because of its high costs, cryogenics generally can be used only in large-scale facilities which can accommodate liquid hydrocarbon recovery.
Membrane-based PSA systems require large pressure differentials across membranes during hydrogen diffusion. This calls for initial compression of the feed prior to contact to the upstream side of polymeric membranes and decompression of the permeate to facilitate final purification steps. Not only are these compression steps expensive, but PSA recovers less feedstream hydrogen and is limited to modest temperatures. U.S. Pat. No. 5,447,559 to Rao discloses a multi-phase (i.e. heterogenous) membrane system used in conjunction with PSA sweep gases.
Many membrane systems have been developed in efforts to efficiently extract target material from feed streams. Some of these membrane systems (U.S. Pat. Nos. 5,030,661, 5,645,626, and 5,725,633) are synthetic based and incorporate polyamides and polyethersulphones. Such organic membranes also have limited temperature tolerance.
Proton-exchange membranes have high proton conductivities, and as such, are currently in development for fuel-cell applications and hydrogen pumps. One such application is disclosed in U.S. Pat. No. 5,094.927 issued to Baucke on Mar. 10, 1992. However, inasmuch as these membranes have relatively low electronic conductivities, they are not viable for hydrogen recovery scenarios, primarily because these membranes require the application of an electric potential to drive proton transport.
Water dissociates into oxygen and hydrogen at high temperatures, and the dissociation increases with increasing temperature:H2O(g)H2+½O2  (1)Because of the small equilibrium constant of this reaction, the concentrations of generated hydrogen and oxygen are very low even at relatively high temperatures, i.e., 0.1 and 0.042% for hydrogen and oxygen, respectively at 1600° C. However, significant amounts of hydrogen or oxygen could be generated at moderate temperatures if the equilibrium were shifted toward dissociation. While hydrogen can also be produced by high-temperature steam electrolysis, the use of a variety of membranes including mixed-conducting membranes offers the advantage of requiring no electric power or electrical circuitry. In considering the above dissociation equation, it appears at first blush that the removal of either hydrogen or oxygen would continue to drive the reaction toward dissociation. However, that is not the entire case as will be hereinafter set forth.
Oxygen derived from reaction (1) can be combined with methane to form syngas:CH4+½O2→CO+H2  (2)
The syngas can be used as a fuel or the H2 can be collected and combined with the H2 from reaction (1). Additionally, the syngas can be subjected to a water gas shift reaction to generate additional hydrogen:
                              2          ⁢          CO                +                  3          ⁢                                    H              2                        ⁢                                          ⟶                catalyst                                                              H                  2                                ⁢                O                                      ⁢            2                    ⁢                      CO            2                          +                  5          ⁢                      H            2                                              (        3        )            